Z-axis elimination in an X-ray laminography system using image magnification for Z plane adjustment

ABSTRACT

Systems and methods for analyzing for images in an x-ray inspection system are provided. One embodiment is a system for analyzing images in an x-ray inspection system. Briefly described, one such system comprises: a means for receiving an image of an object that is generated by an x-ray inspection system, the image of the object having a first field of view (FOV); a means for determining whether the first FOV associated with the image of the object matches a reference FOV corresponding to design data that models the object being inspected by the x-ray inspection system; and a means for modifying the design data based on the difference between the first FOV and the reference FOV.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of copending U.S. utilityapplication entitled, “Z-Axis Elimination in an X-Ray LaminographySystem Using Image Magnification for Z Plane Adjustment,” having Ser.No.09/652,255 and filed Aug. 30, 2000, which is entirely incorporatedherein by reference.

FIELD OF THE INVENTION

[0002] The invention relates generally to the rapid, high resolutioninspection of circuit boards using a computerized laminography system,and in particular, to systems which use electronic means to adjust theZ-axis location of the inspection site with respect to the circuitboard.

BACKGROUND OF THE INVENTION

[0003] Rapid and precise quality control inspections of the solderingand assembly of electronic devices have become priority items in theelectronics manufacturing industry. The reduced size of components andsolder connections, the resulting increased density of components oncircuit boards and the advent of surface mount technology (SMT), whichplaces solder connections underneath device packages where they arehidden from view, have made rapid and precise inspections of electronicdevices and the electrical connections between devices very difficult toperform in a manufacturing environment.

[0004] Many existing inspection systems for electronic devices andconnections make use of penetrating radiation to form images whichexhibit features representative of the internal structure of the devicesand connections. These systems often utilize conventional radiographictechniques wherein the penetrating radiation comprises X-rays. MedicalX-ray pictures of various parts of the human body, e.g., the chest,arms, legs, spine, etc., are perhaps the most familiar examples ofconventional radiographic images. The images or pictures formedrepresent the X-ray shadow cast by an object being inspected when it isilluminated by a beam of X-rays. The X-ray shadow is detected andrecorded by an X-ray sensitive material such as film or other suitablemeans.

[0005] The appearance of the X-ray shadow or radiograph is determinednot only by the internal structural characteristics of the object, butalso by the direction from which the incident X-rays strike the object.Therefore, a complete interpretation and analysis of X-ray shadowimages, whether performed visually by a person or numerically by acomputer, often requires that certain assumptions be made regarding thecharacteristics of the object and its orientation with respect to theX-ray beam. For example, it is often necessary to make specificassumptions regarding the shape, internal structure, etc. of the objectand the direction of the incident X-rays upon the object. Based on theseassumptions, features of the X-ray image may be analyzed to determinethe location, size, shape, etc., of the corresponding structuralcharacteristic of the object, e.g., a defect in a solder connection,which produced the image feature. These assumptions often createambiguities which degrade the reliability of the interpretation of theimages and the decisions based upon the analysis of the X-ray shadowimages. One of the primary ambiguities resulting from the use of suchassumptions in the analysis of conventional radiographs is that smallvariations of a structural characteristic within an object, such as theshape, density and size of a defect within a solder connection, areoften masked by the overshadowing mass of the solder connection itselfas well as by neighboring solder connections, electronic devices,circuit boards and other objects. Since the overshadowing mass andneighboring objects are usually different for each solder joint, it isextremely cumbersome and often nearly impossible to make enoughassumptions to precisely determine shapes, sizes and locations of solderdefects within individual solder joints.

[0006] In an attempt to compensate for these shortcomings, some systemsincorporate the capability of viewing the object from a plurality ofangles. One such system is described in U.S. Pat. No. 4,809,308 entitled“METHOD & APPARATUS FOR PERFORMING AUTOMATED CIRCUIT BOARD SOLDERQUALITY INSPECTIONS”, issued to Adams et al. The additional views enablethese systems to partially resolve the ambiguities present in the X-rayshadow projection images. However, utilization of multiple viewingangles necessitates a complicated mechanical handling system, oftenrequiring as many as five independent, non-orthogonal axes of motion.This degree of mechanical complication leads to increased expense,increased size and weight, longer inspection times, reduced throughput,impaired positioning precision due to the mechanical complications, andcalibration and computer control complications due to thenon-orthogonality of the axes of motion.

[0007] Many of the problems associated with the conventional radiographytechniques discussed above may be alleviated by producingcross-sectional images of the object being inspected. Tomographictechniques such as laminography and computed tomography (CT) have beenused in medical applications to produce cross-sectional or body sectionimages. In medical applications, these techniques have met withwidespread success, largely because relatively low resolution on theorder of one or two millimeters (0.0425 to 0.08 inches) is satisfactoryand because speed and throughput requirements are not as severe as thecorresponding industrial requirements.

[0008] In the case of electronics inspection, and more particularly, forinspection of electrical connections such as solder joints, imageresolution on the order of several micrometers, for example, 20micrometers (0.0008 inches) is preferred. Furthermore, an industrialsolder joint inspection system must generate multiple images per secondin order to be practical for use on an industrial production line.Laminography systems which are capable of achieving the speed andaccuracy requirements necessary for electronics inspection are describedin the following patents: 1) U.S. Pat. No. 4,926,452 entitled “AUTOMATEDLAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICS”, issued to Baker etal.; 2) U.S. Pat. No. 5,097,492 entitled “AUTOMATED LAMINOGRAPHY SYSTEMFOR INSPECTION OF ELECTRONICS”, issued to Baker et al.; 3) U.S. Pat. No.5,081,656 entitled “AUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OFELECTRONICS”, issued to Baker et al.; 4) U.S. Pat. No. 5,291,535entitled “METHOD AND APPARATUS FOR DETECTING EXCESS/INSUFFICIENT SOLDERDEFECTS”, issued to Baker et al.; 5) U.S. Pat. No. 5,621,811 entitled“LEARNING METHOD AND APPARATUS FOR DETECTING AND CONTROLLING SOLDERDEFECTS”, issued to Roder et al.; 6) U.S. Pat. No. 5,561,696 “METHOD &APPARATUS FOR INSPECTING ELECTRICAL CONNECTIONS”, issued to Adams etal.; 7) U.S. Pat. No. 55,199,054 entitled “METHOD AND APPARATUS FOR HIGHRESOLUTION INSPECTION OF ELECTRONIC ITEMS”, issued to Adams et al.; 8)U.S. Pat. No. 5,259,012 entitled “LAMINOGRAPHY SYSTEM AND METHOD WITHELECTROMAGNETICALLY DIRECTED MULTIPATH RADIATION SOURCE”, issued toBaker et al.; 9) U.S. Pat. No. 5,583,904 entitled “CONTINUOUS LINEARSCAN LAMINOGRAPHY SYSTEM AND METHOD”, issued to Adams; and 10) U.S. Pat.No. 5,687,209 entitled “AUTOMATIC WARP COMPENSATION FOR LAMINOGRAPHICCIRCUIT BOARD INSPECTION”, issued to Adams. The entirety of each of theabove referenced patents is hereby incorporated herein by reference.

[0009] Several of the above referenced patents disclose devices andmethods for the generation of cross-sectional images of test objects ata fixed or selectable cross-sectional image focal plane. In thesesystems, an X-ray source system and an X-Ray detector system areseparated in the Z-axis direction by a fixed distance and thecross-sectional image focal plane is located at a predetermined specificposition on the Z-axis which is intermediate the Z-axis locations of theX-ray source system and the X-ray detector system. The X-ray detectorsystem collects data from which a cross-sectional image of features inthe test object, located at the cross-sectional image focal plane, canbe formed. In systems having a fixed cross-sectional image focal plane,it is necessary to postulate that the features desired to be imaged arelocated in the fixed cross-sectional image focal plane at thepredetermined specific position along the Z-axis. Thus, in thesesystems, it is essential that the positions of the fixed cross-sectionalimage focal plane and the plane with respect to the object which isdesired to be imaged, be configured to coincide at the same positionalong the Z-axis. If this condition is not met, then the desired imageof the selected feature of the test object will not be acquired.Instead, a cross-sectional image of a plane with respect to the testobject which is either above or below the plane which includes theselected feature will be acquired. Thus, mechanical motion of the testobject along the Z-axis is often used to position the desired plane withrespect to the test object which is to be imaged at the position of thefixed cross-sectional image focal plane of the inspection system.

[0010] Since the laminographic image area (e.g., 2-3 cm²) of a typicallaminography system is substantially smaller than the area of a typicalcircuit board (e.g., 150-1,500 cm²), a complete inspection of a circuitboard includes multiple laminographic images, which, if pieced togetherwould form an image of the entire circuit board or selected regions ofthe circuit board. Thus, in addition to having a Z-axis mechanicalpositioning system for placing the test object (circuit board) at aspecific location along the Z-axis, a typical high resolutionlaminography system also includes X-axis and Y-axis mechanicalpositioning systems for placing the test object at specific locationsalong the X and Y axes. This is frequently achieved by supporting thetest object on a mechanical handling system, such as an X, Y, Zpositioning table. The table is then moved to bring the desired regionsof the test object into the laminographic image area of the laminographysystem. Movement in the X and Y directions locates the region of thetest object to be examined, while movement in the Z direction moves thetest object up and down to select the plane with respect to the testobject where the cross sectional image is to be taken. As usedthroughout this document, the phrase “board view” will be used to referto the laminographic image of a particular region or area of a circuitboard identified by a specific X, Y coordinate of the circuit board.Thus, each “board view” includes only a portion of the circuit board.

[0011] Many inspections require that some of the board views includemultiple images at different Z-axis levels of the circuit board. Thismay be accomplished by physically moving the circuit board up or down inthe Z-axis direction using the X, Y, Z mechanical positioning table.However, this additional mechanical motion along the Z-axis directioncan also lead to increased expense, increased size and weight, longerinspection times, reduced throughput, reduced image resolution andaccuracy due to mechanical vibrations and impaired Z-axis positioningprecision due to mechanical complications.

[0012] An alternative to mechanical Z-axis positioning is disclosed inU.S. Pat. No. 5,259,012 entitled “LAMINOGRAPHY SYSTEM AND METHOD WITHELECTROMAGNETICALLY DIRECTED MULTIPATH RADIATION SOURCE”, issued toBaker et al. This patent describes a laminography system whichelectronically shifts the Z-axis location of the image plane withrespect to the test object. In this device, the test object isinterposed between a rotating X-Ray source and a synchronized rotatingX-ray detector. A focal plane with respect to the test object is imagedonto the detector so that a cross-sectional image of a layer of the testobject which coincides with the image focal plane is produced. The X-raysource is produced by deflecting an electron beam onto a target anode.The target anode emits X-ray radiation where the electrons are incidentupon the target anode. The electron beam is produced by an electron gunwhich includes X and Y deflection coils for deflecting the electron beamin the X and Y directions. The X and Y deflection coils cause the X-raysource to rotate in a circular trace path. The voltages applied to the Xand Y deflection coils are adjusted to change the radius of the circulartrace path on the target anode resulting in a change in the Z-axislocation of the image plane with respect to the test object. Acharacteristic of this type of electronic Z-axis positioning system isthat images produced at different Z-axis positions have differentmagnification factors. The different magnification factors of the imagescomplicates the analysis of the multiple images acquired during acomplete inspection of the circuit board.

[0013] In summary, the magnification of multiple board views atdifferent Z levels is not changed when using systems of the typepreviously described wherein the X-ray source and detector are fixed atspecific locations along the Z-axis and the circuit board is moved inthe Z-axis direction to obtain laminographic images at the different Zlevels of the circuit board. Alternatively, the magnification ofdifferent Z level board views does change with each change in Z levelwhen using the previously described systems which electronically changethe radius of the X-Ray source to obtain lamingraphic images atdifferent Z levels of the circuit board. The different magnificationsfor different Z level board views in these systems presents difficultiesin analyzing the images thus obtained.

[0014] The present invention provides improvements which address theabove listed specific problems. The present invention advantageouslyincludes ease of use and improved accuracy of Z elevation determination,resulting in an improved technique for producing high resolution crosssectional images of electrical connections.

SUMMARY OF THE INVENTION

[0015] The present invention comprises an improved computerizedlaminography system which accurately compensates for variablemagnifications of different Z level board views in an efficient manner.This feature makes it feasible to eliminate the Z-axis mechanical motionof the circuit board along the Z-axis direction. Elimination of theZ-axis mechanical motion improves speed of the inspection as well asreliability of the inspection system.

[0016] As used throughout this document, the phrase “field of view” or“FOV” will be used to refer to the size of a particular region or areaof a circuit board which is included in a laminographic image of thatparticular region or area of the circuit board. For example, oneparticular configuration of the present invention has two presetmagnification factors. A first magnification factor of 4.75 has a FOV of0.8 inch×0.8 inch and an image size of 3.8 inches×3.8 inches. Thus, aboard view at a particular x, y location of the circuit board at amagnification of 4.75 refers to a 3.8 inches×3.8 inches image of a 0.8inch×0.8 inch region of the circuit board centered at location x, y onthe circuit board. A second magnification factor of 19 has a FOV of 0.2inch×0.2 inch and an image size of 3.8 inches×3.8 inches. Thus, a boardview at a particular x, y location of the circuit board at amagnification of 19 refers to a 3.8 inches×3.8 inches image of a 0.2inch×0.2 inch region of the circuit board centered at location x, y onthe circuit board. Thus, four board views at the magnification of 19,each board view having a FOV of 0.2 inch×0.2 inch, are required to imagethe single corresponding board view at the magnification of 4.75, eachboard view having a FOV of 0.8 inch×0.8 inch. In terms of FOV, the FOVof the system operating at a magnification factor of 4.75 is 4 timeslarger than the FOV of the system operating at a magnification factor of19.

[0017] As described above, in addition to changing the magnification ofthe image, another side effect of changing the Z-axis location of theimage plane electronically as opposed to mechanically is that the fieldof view (FOV) for different Z-axis locations of the image plane alsochanges as the magnification changes. In systems having a fixed Z-axislocation of the image plane, the magnification and FOV are not dependenton which Z-level of the circuit board is being imaged since differentZ-levels of the circuit board are mechanically positioned at the samefixed Z-axis location of the image plane of the system.

[0018] There are several ways that this change in FOV with magnificationcan be accounted for and corrected in analyzing the images. In circuitboard inspection systems, CAD data which describes the circuit boardbeing inspected is utilized during the acquisition and analysis of theimages of the circuit board. Thus, a first technique for compensatingfor variable image magnification factors and FOV's may be accomplishedby magnifying or shrinking the acquired images to a “nominal” size(“nominal” being defined by a base FOV). Numerous algorithms for doingthis are well documented in the technical literature. However, thesetechniques tend to be CPU intensive and may affect throughput of thesystem. A second and preferred technique for compensating for variableimage magnification factors and FOV's may be accomplished moreefficiently by using on-the-fly CAD data manipulation and on-the-fly FOVadjustments during the analysis of the images.

[0019] In a first aspect, the present invention is a device forinspecting electrical connections on a circuit board comprising: asource of X-rays which emits X-rays through the electrical connectionfrom a plurality of positions centered about a first radius and a secondradius; an X-ray detector system positioned to receive the X-raysproduced by the source of X-rays which have penetrated the electricalconnection, the X-ray detector system further comprising an output whichemits data signals; an image memory which combines the detector datasignals to form an image database which contains information sufficientto form a first cross-sectional image of a cutting plane of theelectrical connection at a first image plane at a first Z-axis locationcorresponding to the first X-ray source radius and a secondcross-sectional image of a cutting plane of the electrical connection ata second image plane at a second Z-axis location corresponding to thesecond X-ray source radius; and a processor which controls theacquisition and formation of the cross-sectional images and analyzes thecross-sectional images, the image processor further comprising: astorage area for storing CAD data which describes a firstcross-sectional design of the electrical connection at the first imageplane at the first Z-axis location and CAD data for a secondcross-sectional design of the electrical connection at the second imageplane at the second Z-axis location; and a CAD data calculator sectionwhich determines a variance between the first cross-sectional image atthe first image plane and the second cross-sectional image at the secondimage plane and uses the variance to modify, on an as-needed basis,portions of the CAD data which describe said electrical connection atthe second image plane at the second Z-axis location thereby generatingmodified CAD data for the second image plane which describes theelectrical connection at the second image plane as represented by thesecond cross-sectional image. In some configurations, the first crosssectional image has a first field of view and the second cross sectionalimage has a second field of view and the variance between the firstcross-sectional image and the second cross-sectional image is determinedby comparing the second field of view to the first field of view. Insome configurations, the first cross sectional image has a firstmagnification factor and the second cross sectional image has a secondmagnification factor and the variance between the first cross-sectionalimage and the second cross-sectional image is determined by comparingthe second magnification factor to the first magnification factor. Insome configurations, the source of X-rays comprises a plurality of X-raysources. In some configurations, the X-ray detector system comprises aplurality of X-ray detectors. In some configurations, the processorfurther comprises an image section which produces the cross-sectionalimages of the electrical connection from the image database.

[0020] A second aspect of the present invention includes a method foranalyzing laminographic images of an object at multiple Z-axis levelswithin the object comprising the steps of: determining a referenceZ-axis position Z₁, corresponding to a first Z level in the object;acquiring a first cross sectional image of the object at the referenceZ-axis position Z₁, which corresponds to the first Z level in the objectand a second cross sectional image of the object at a second Z-axisposition Z₂ which corresponds to a second Z level in the object;providing first Z level design data which describes the object andspecific features within the object at the first Z level of the objectand second Z level design data which describes the object and specificfeatures within the object at the second Z level of the object;determining a variance factor which represents a difference between thefirst cross sectional image of the object at the first Z level and thesecond cross sectional image of the object at the second Z level; andmodifying in real time or near real time, one or more portions of thesecond Z level design data with the variance factor while comparing thesecond cross sectional image of the object at the second Z level withthe real time or near real time modified second Z level design data. Insome implementations of the method, the first cross sectional image hasa first field of view and the second cross sectional image has a secondfield of view and the variance factor which represents a differencebetween the first cross-sectional image and the second cross-sectionalimage is determined by comparing the second field of view to the firstfield of view. In some implementations of the method, the first crosssectional image has a first magnification factor and the second crosssectional image has a second magnification factor and the variancefactor which represents a difference between the first cross-sectionalimage and the second cross-sectional image is determined by comparingthe second magnification factor to the first magnification factor.

[0021] A third aspect of the present invention includes a method forinspecting an electrical connection on a circuit board comprising:determining a first Z-axis position Z₁, corresponding to a first Z levelin the electrical connection; acquiring a first cross sectional image ofthe electrical connection at the first Z-axis position Z₁, whichcorresponds to the first Z level in the electrical connection and asecond cross sectional image of the electrical connection at a secondZ-axis position Z₂ which corresponds to a second Z level in theelectrical connection, wherein the first cross sectional image has afirst magnification factor and the second cross sectional image has asecond magnification factor; providing first Z level design data whichdescribes the electrical connection and specific design features withinthe electrical connection at the first Z level of the electricalconnection and second Z level design data which describes the electricalconnection and specific design features within the electrical connectionat the second Z level of the electrical connection; comparing the firstand second magnification factors to determine a first field of viewcorrection factor; and modifying in real time or near real time, one ormore portions of the second Z level design data with the first field ofview correction factor while comparing the second cross sectional imageof the electrical connection at the second Z level with the real time ornear real time modified second Z level design data.

[0022] Some implementations of this method further comprise: providingthird Z level design data which describes the electrical connection andspecific design features within the electrical connection at a third Zlevel of the electrical connection; acquiring a third cross sectionalimage of the electrical connection at a third Z-axis position Z₃ whichcorresponds to the third Z level in the electrical connection whereinthe third cross sectional image has a third magnification factor;comparing the first and third magnification factors to determine asecond field of view correction factor; and modifying in real time ornear real time, one or more portions of the third Z level design datawith the second field of view correction factor while comparing thethird cross sectional image of the electrical connection at the third Zlevel with the real time or near real time modified third Z level designdata.

[0023] These and other characteristics of the present invention willbecome apparent through reference to the following detailed descriptionof the preferred embodiments and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a schematic representation of a laminography systemillustrating the principles of the technique.

[0025]FIG. 2a shows an object having an arrow, a circle and a crossembedded in the object at three different planar locations.

[0026]FIG. 2b shows a laminograph of the object in FIG. 2a focused onthe plane containing the arrow.

[0027]FIG. 2c shows a laminograph of the object in FIG. 2a focused onthe plane containing the circle.

[0028]FIG. 2d shows a laminograph of the object in FIG. 2a focused onthe plane containing the cross.

[0029]FIG. 2e shows a conventional, two-dimensional X-ray projectionimage of the object in FIG. 2a.

[0030]FIG. 3a is a diagrammatic cross-sectional view of a circuit boardinspection laminography system showing how the laminographic image isformed and viewed by a camera.

[0031]FIG. 3b shows a top view enlargement of an inspection region shownin FIG. 3a.

[0032]FIG. 3c is a perspective view of the circuit board inspectionlaminography system shown in FIG. 3a.

[0033]FIGS. 4 and 5 are schematic views of a laminography system inaccordance with the present invention.

[0034]FIGS. 6 and 7 illustrate the manner in which a laminographicsystem in accordance with the present invention is utilized to produce aZ-axis shaft of the imaged region of the object plane with respect tothe object.

[0035]FIG. 8 illustrates a possible configuration of a X-ray targetanode which may be used with the present invention.

[0036]FIG. 9 illustrates how the magnification of an image is related tothe distance between an image plane and an X-ray detector.

[0037]FIG. 10 illustrates how the FOV and magnification changes in afirst implementation of the present invention.

[0038]FIG. 11 shows a perspective view of the test object 10 shown inFIG. 2a mounted on a circuit board.

[0039]FIG. 12 shows a cross sectional view of the test object 10 mountedon the circuit board shown in FIG. 11.

[0040]FIGS. 13A, 13B, and 13C show CAD data for the test object 10 shownin FIGS. 2, 11 and 12.

[0041]FIGS. 14A, 14B, and 14C show laminographic images of the testobject 10 shown in FIGS. 2, 11 and 12 corresponding to the CAD data inFIGS. 13A, 13B and 13C.

[0042]FIG. 15 is a flow chart showing a process for performingon-the-fly CAD data manipulations for the analysis of laminographicimages.

REFERENCE NUMERALS IN DRAWINGS

[0043]  10 object under inspection  20 source of X-rays  30 X-raydetector  40 common axis of rotation  50 central ray  52 X-rayprojection  60 image plane in object 10  60a arrow image plane  60bcircle image plane  60c cross image plane  62 plane of source of X-rays 64 plane of X-ray detecto  70 point of intersection  81 arrow testpattern  82 circle test pattern  83 cross test pattern 100 image ofarrow 81 102 blurred region 110 image of circle 82 112 blurred region120 image of cross 83 122 blurred region 130 image of arrow 81 132 imageof circle 82 134 image of cross 83 200 X-ray tube 210 printed circuitboard 212 electronic components 214 electrical connections 220 supportfixture 230 positioning table 240 rotating X-ray detector 250fluorescent screen 252 first mirror 254 second mirror 256 turntable 258camera 260 feedback system 262 input connection 263 sensor 264 outputconnection 265 position encoder 270 computer 276 input line 278 outputline 280 rotating source spot 281 deflection coils 282 X-rays 283 regionof circuit board 284 X-rays 285 rotating electron beam 286 light 287target anode 290 granite support table 292 load/unload port 294 operatorstation 296 laser range finder 310 laminography system 312 source ofX-rays 314 object 315 analysis system 316 rotating X-ray detector 318electron gun 320 electrodes 322 coils 324 target anode 330 electron beam332 X-ray spot 334 X-rays 340 fluorescent screen 342 first mirror 344second mirror 346 turntable 348 platform 349 granite table 350 axis 352plane 356 camera 357 video terminal 360 focus coil 362 steeringyoke/deflection coil 363 look up table (LUT) 410 first plane of object414 412 second plane of object 414 414 object under inspection 416 coneof X-rays 418 cone of X-rays 420 image of cross 472 424 scan circle withradius R1 425 scan circle with radius R2 426 cone of X-rays 428 cone ofX-rays 430 image of arrow 470 470 arrow test pattern 472 cross testpattern 550 target anode 560 X-rays 620 circuit board 640 Field of View(FOV) 720cd CAD data for test object 10 at Level 1 720id image data fortest object 10 at Level 1 750cd CAD data for test object 10 at Level 2750id image data for test object 10 at Level 2 780cd CAD data for testobject 10 at Level 3 780id image data for test object 10 at Level 3 810flow chart process block 820 flow chart process block 830 flow chartprocess block 840 flow chart process block 850 flow chart process block860 flow chart process block

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] As used throughout, the term “radiation” refers toelectromagnetic radiation, including but not limited to the X-ray, gammaand ultraviolet portions of the electromagnetic radiation spectrum.

[0045] Cross-sectional Image Formation

[0046]FIG. 1 shows a schematic representation of a typical laminographicgeometry used with the present invention. An object 10 underexamination, for example, a circuit board, is held in a stationaryposition with respect to a source of X-rays 20 and an X-ray detector 30.Synchronous rotation of the X-ray source 20 and detector 30 about acommon axis 40 causes an X-ray image of the plane 60 with respect to theobject 10 to be formed on the detector 30. The image plane 60 issubstantially parallel to planes 62 and 64 defined by the rotation ofthe source 20 and detector 30, respectively. The image plane 60 islocated at an intersection 70 of a central ray 50 from the X-ray source20 and the common axis of rotation 40. This point of intersection 70acts as a fulcrum for the central ray 50, thus causing an in-focuscross-sectional X-ray image of the object 10 at the plane 60 to beformed on detector 30 as the source and detector synchronously rotateabout the intersection point 70. Structure with respect to the object 10which lies outside of plane 60 forms a blurred X-ray image on detector30.

[0047] In the laminographic geometry shown in FIG. 1, the axis ofrotation of the radiation source 20 and the axis of rotation of thedetector 30 are coaxial. However, it is not necessary that these axes ofrotation of the radiation source and the detector 30 be coaxial. Theconditions of laminography are satisfied and a cross-sectional image ofthe layer 60 will be produced as long as the planes of rotation 62 and64 are mutually parallel, and the axes of rotation of the source and thedetector are mutually parallel and fixed in relationship to each other.Coaxial alignment reduces the number of constraints upon the mechanicalalignment of the apparatus.

[0048]FIGS. 2a-2 e show laminographs produced by the above describedlaminographic technique. The object 10 shown in FIG. 2a has testpatterns in the shape of an arrow 81, a circle 82 and cross 83 embeddedwithin the object 10 in three different planes 60 a, 60 b and 60 c,respectively.

[0049]FIG. 2b shows a typical laminograph of object 10 formed ondetector 30 when the point of intersection 70 lies in plane 60 a of FIG.2a. An image 100 of arrow 81 is in sharp focus, while the images ofother features within the object 10, such as the circle 82 and cross 83form a blurred region 102 which does not greatly obscure the arrow image100.

[0050] Similarly, when the point of intersection 70 lies in plane 60 b,an image 110 of the circle 82 is in sharp focus as seen in FIG. 2c. Thearrow 81 and cross 83 form a blurred region 112.

[0051]FIG. 2d shows a sharp image 120 formed of the cross 83 when thepoint of intersection 70 lies in plane 60 c. The arrow 81 and circle 82form blurred region 122.

[0052] For comparison, FIG. 2e shows an X-ray shadow image of object 10formed by conventional projection radiography techniques. This techniqueproduces sharp images 130, 132 and 134 of the arrow 81, circle 82 andcross 83, respectively, which overlap one another. FIG. 2e vividlyillustrates how multiple characteristics contained within the object 10may create multiple overshadowing features in the X-ray image whichobscure individual features of the image.

[0053]FIG. 3a illustrates a schematic diagram of a typical laminographicapparatus used with the present invention. In this configuration, anobject under inspection is a printed circuit board 210 having multipleelectronic components 212 mounted on the board 210 and electricallyinterconnected via electrical connections 214 (See FIG. 3b). Typically,the electrical connections 214 are formed of solder. However, variousother techniques for making the electrical connections 214 are well knowin the art and even though the invention will be described in terms ofsolder joints, it will be understood that other types of electricalconnections 214 including, but not limited to, conductive epoxy,mechanical, tungsten and eutectic bonds may be inspected utilizing theinvention. FIG. 3b, which is a top view enlargement of a region 283 ofthe circuit board 210, more clearly shows the components 212 and solderjoints 214.

[0054] The laminographic apparatus acquires cross-sectional images ofthe solder joints 214 using the previously described laminographicmethod or other methods capable of producing equivalent cross-sectionalimages. The cross sectional images of the solder joints 214 areautomatically evaluated to determine their quality. Based on theevaluation, a report of the solder joint quality is presented to theuser.

[0055] The laminographic apparatus, as shown in FIG. 3a, comprises anX-ray tube 200 which is positioned adjacent printed circuit board 210.The circuit board 210 is supported by a fixture 220. The fixture 220 isattached to a positioning table 230 which is capable of moving thefixture 220 and board 210 along three mutually perpendicular axes, X, Yand Z. A rotating X-ray detector 240 comprising a fluorescent screen250, a first mirror 252, a second mirror 254 and a turntable 256 ispositioned adjacent the circuit board 210 on the side opposite the X-raytube 200. A camera 258 is positioned opposite mirror 252 for viewingimages reflected into the mirrors 252, 254 from fluorescent screen 250.A feedback system 260 has an input connection 262 from a sensor 263which detects the angular position of the turntable 256 and an outputconnection 264 to X and Y deflection coils 281 on X-ray tube 200. Aposition encoder 265 is attached to turntable 256. The position sensor263 is mounted adjacent encoder 265 in a fixed position relative to theaxis of rotation 40. The camera 258 is connected to a computer 270 viaan input line 276. The computer 270 includes the capability to performhigh speed image analysis. An output line 278 from the computer 270connects the computer to positioning table 230. A laser range finder 296is positioned adjacent the circuit board 210 for creating a Z-map of thesurface of the circuit board 210.

[0056] A perspective view of the laminographic apparatus is shown inFIG. 3c. In addition to the X-ray tube 200, circuit board 210,fluorescent screen 250, turntable 256, camera 258, positioning table 230and computer 270 shown in FIG. 3a, a granite support table 290, aload/unload port 292 and an operator station 294 are shown. The granitetable 290 provides a rigid, vibration free platform for structurallyintegrating the major functional elements of the laminographicapparatus, including but not limited to the X-ray tube 200, positioningtable 230 and turntable 256. The load/unload port 292 provides a meansfor inserting and removing circuit boards 210 from the machine. Theoperator station 294 provides an input/output capability for controllingthe functions of the laminographic apparatus as well as forcommunication of inspection data to an operator.

[0057] In operation of the laminographic apparatus as shown in FIGS. 3aand 3 c, high resolution, cross-sectional X-ray images of the solderjoints 214 connecting components 212 on circuit board 210 are acquiredusing the X-ray laminographic method previously described in referenceto FIGS. 1 and 2. Specifically, X-ray tube 200, as shown in FIG. 3a,comprises a rotating electron beam spot 285 which produces a rotatingsource 280 of X-rays 282. The X-ray beam 282 illuminates a region 283 ofcircuit board 210 including the solder joints 214 located within region283. X-rays 284 which penetrate the solder joints 214, components 212and board 210 are intercepted by the rotating fluorescent screen 250.

[0058] Dynamic alignment of the position of the X-ray source 280 withthe position of rotating X-ray detector 240 is precisely controlled byfeedback system 260. The feedback system correlates the position of therotating turntable 256 with calibrated X and Y deflection values storedin a look-up table (LUT). Drive signals proportional to the calibrated Xand Y deflection values are transmitted to the steering coils 281 on theX-ray tube 200. In response to these drive signals, steering coils 281deflect electron beam 285 to locations on an annular shaped target anode287 such that the position of the X-ray source spot 280 rotates insynchronization with the rotation of detector 240 in the mannerpreviously discussed in connection with FIG. 1. X-rays 284 whichpenetrate the board 210 and strike fluorescent screen 250 are convertedto visible light 286, thus creating a visible image of a single planewithin the region 283 of the circuit board 210. The visible light 286 isreflected by mirrors 252 and 254 into camera 258. Camera 258 typicallycomprises a low light level closed circuit TV (CCTV) camera whichtransmits electronic video signals corresponding to the X-ray andvisible images to the computer 270 via line 276. The image analysisfeature of computer 270 analyzes and interprets the image to determinethe quality of the solder joints 214.

[0059] Computer 270 also controls the movement of positioning table 230and thus circuit board 210 so that different regions of circuit board210 may be automatically positioned within inspection region 283.

[0060] The laminographic geometry and apparatus shown and described withreference to FIGS. 1-3 are typical of that which may be used inconjunction with the present invention. However, specific details ofthese systems are not critical to the practice of the present invention,which addresses an alternate and/or additional technique for adjustingthe Z-axis location of the image plane within the circuit board 210. Forexample, the number of computers and delegation of tasks to specificcomputers may vary considerably from system to system as may thespecific details of the X-ray source, detector, circuit boardpositioning mechanism, etc.

[0061] Electronic Z-axis Laminography System

[0062]FIG. 4 illustrates a schematic diagram of a laminography system310 in accordance with the present invention. The system 310 comprises asource of X-rays 312 positioned above an object 314 to be viewed, and arotating X-ray detector 316, positioned below the object 314, oppositethe X-ray source 312. The object 314 may, for example, be an electronicitem such as a circuit board, a manufactured item such as an aircraftpart, a portion of a human body, etc.

[0063] The invention acquires X, Y plane cross-sectional images of theobject 314 under inspection using multipath laminography geometrieswhich enables multiple locations of the object 314 to be sequentiallyviewed without requiring mechanical movement of the object 314. Movementin various scan circles produces laminographs at the desired X, Ycoordinate locations and various Z planes without the need formechanical movement of the viewed object 314. In one embodiment, theinvention may be interfaced with an analysis system 315 whichautomatically evaluates the cross-section image generated by the system310 and provides a report to the user that indicates the results of theevaluation.

[0064] The source 312 is positioned adjacent the object 314, andcomprises an electron gun 318, a set of electrodes for electron beamacceleration and focus 320, a focus coil 360, and a steering yoke ordeflection coil 362, and a substantially flat target anode 324. Anelectron beam 330 emitted from the electron gun 318 is incident upon thetarget 324, producing an X-ray spot 332 which serves as an approximatelypoint source of X-rays 334. The X-rays 334 originate in the target 324from the point where the electron beam 330 impinges upon the target 324and, as described below, illuminate various regions of the object 314.

[0065] The object 314 is typically mounted on a platform 348 which maybe affixed to a granite table 349, so as to provide a rigid,vibration-free platform for structurally integrating the functionalelements of the system 310, including the X-ray source 312 and turntable346. It is also possible that the platform 348 comprises a positioningtable that is capable of moving the object 314 relatively largedistances along three mutually perpendicular axes X, Y, and Z.

[0066] The rotating X-ray detector 316 comprises a fluorescent screen340, a first mirror 342, a second mirror 344, and a turntable 346. Theturntable 346 is positioned adjacent to the object 314, on the sideopposite to the X-ray source 312. A camera 356 is positioned oppositethe mirror 344, for viewing images reflected into the mirrors 342, 344from the fluorescent screen 340. The camera 356 typically comprises alow light level closed circuit television or CCD camera that produces avideo image of the X-ray image formed on the fluorescent screen 340. Thecamera 356 may, for example, be connected to a video terminal 357 sothat an operator may observe the image appearing on the detector 340.The camera 356 may also be connected to the image analysis system 315.

[0067] The laminography system 310 is advantageously encased by asupporting chassis (not shown) which acts to prevent undesired emissionsof X-rays, as well as facilitating the structural integration of themajor elements of the system 310.

[0068] In operation, X-rays 334 produced by the X-ray source 312illuminate and penetrate regions of the object 314 and are interceptedby the screen 340. Synchronous rotation of the X-ray source 312 anddetector 316 about an axis 350 causes an X-ray image of a plane 352 (seeFIG. 5) within the object 314 to be formed on the detector 316. Althoughthe axis of rotation 350 illustrated is the common axis of rotation forboth the source 312 and detector 316, one skilled in the art willrecognize that it is not necessary for the axes of rotation to becollinear. In practice, it is sufficient that the axes of rotation beparallel. X-rays 334 which penetrate the object 314 and strike thescreen 340 are converted into visible light reflected by the mirrors342, 344 and into the camera 356.

[0069] Referring to FIG. 5, the electron beam 330 is emitted from theelectron gun 318 and travels in a region between the electrodes 320 andsteering coils 322. The electrodes 320 and coils 322 produceelectromagnetic fields which interact with the electron beam 330 tofocus and direct the beam 330 onto the target 324 forming an electronbeam spot 332 from which X-rays are emitted. Preferably, the size of theelectron beam spot 332 on the target is on the order of 0.02 to 10microns in diameter. The steering coils 322 enable the X-ray source 312to provide X-rays 334 from the X-ray spots 332 wherein the location ofthe spots 332 move in a desired pattern around the target 324.Preferably, the steering coils 322 comprise separate X and Yelectromagnetic deflection coils 360, 362 which deflect the electronbeam 330 discharged from the electron gun 318 in the X and Y directions,respectively. Electrical current flowing in the steering yoke 362creates a magnetic field which interacts with the electron beam 330causing the beam 330 to be deflected. However, one skilled in the artwill also recognize that electrostatic deflection techniques could alsobe used to deflect the electron beam 330.

[0070] Preferably, a LUT 363 outputs voltage signals which, when appliedto the X and Y deflection coils 360, 362 cause the electron beam spot332 to rotate, thus producing a circular pattern on the surface of thetarget 324. In one embodiment, the LUT 363 provides the output voltagesin response to addressing signals from a master computer (not shown)which may be included within the image analysis system 315. The outputvoltages are advantageously predetermined using a calibration techniquewhich correlates the position of the turntable 346, and the position ofthe X-ray beam spot 332.

[0071] The present invention provides a method and apparatus forprocessing laminographic images of various Z-axis levels of the object314 which requires little or no physical movement of the object 314 orthe supporting table 348. In accordance with the present invention,desired Z-axis levels of the object are brought within the field of viewof the system electronically as opposed to mechanically. This isaccomplished by moving the location of the pattern traced by the X-raybeam spot 332 on the target 324. In this manner, various Z-axis levelsof the object 314 are brought within the field of view and images areproduced of a specific Z-axis level of the object coinciding with thefield of view. In accordance with the present invention, the voltagesapplied to the X and Y deflection coils 360, 362 are varied in order toproduce rotating X-ray beam paths of distinct radii having distinct x, ylocations on the target 324.

[0072] Referring to FIG. 6 and FIG. 7, the present invention furtherprovides a laminography system having a geometry which can be utilizedto effect a shift or change in the Z-axis position of the object plane60 (see FIG. 1) within a test object 414 without moving the test object.FIG. 6 illustrates an object 414 having the patterns of an arrow 470 anda cross 472 located therein. The cross pattern 472 is located in a firstplane 410 and the arrow pattern 470 is located in a second plane 412,wherein the first plane 410 lies above and is parallel to the secondplane 412. The X-ray beam spot 332 traces a scan circle 424 having aradius R1, defining a family of cones including cones 416, 418.

[0073] The intersection of the cones formed as the X-ray beam spot 332travels around the circle 424, including cones 416, 418, forms an imageregion substantially centered about the cross pattern 472, such that thefirst plane 410 is defined as the object plane 60. As the X-ray spot 332and detector 316 rotate in synchronization, a distinct image 420 of thecross pattern 472 is produced on the surface of the detector 316. Theimage of the arrow 470, which lies in the second plane 412 and isoutside the object plane 410 defined by the cones 416, 418, is notstationary on the detector 316 during the entire rotation of thedetector 316 and thus, appears blurred.

[0074]FIG. 7 illustrates that by adjusting the gain of the voltagesoutput from the LUT 363 to the deflection coils 360, 362, therebychanging the amplitude of sine and cosine signals driving the coils, theradii of the scan circles 424, 425 traced by the X-ray spot 332 can bevaried to produce images of regions within distinct Z-axis planes in theobject 414. With the adjustment of the gain applied to the output fromthe LUT 363, the scan circle 424 is increased in radius by a value ΔR toa radius R2, thereby forming a scan circle 425 defining a second familyof cones including the cones 426, 428. Because of the larger radius R2of the second scan circle 425, the set of points defined by theintersection of a second family of cones, including cones 426, 428, isdisplaced in the negative Z direction relative to the region imaged whenthe X-ray source 332 follows the path 424 (FIG. 6). Thus, the objectplane 60 is lowered by an amount ΔZ to the second plane 412, and theimage region is substantially centered about the arrow pattern 470. Asthe X-ray spot 332 and detector 316 rotate, a distinct image 430 of thearrow pattern 470 is then produced on the detector 316, while the imageof the cross pattern 472, lying outside the object plane 412, appearsblurred. The amplitude of the gain adjustment made to the voltagesapplied to the deflection coils 360, 362 is proportional to thedirection and amount of the shift ΔZ in the position of the object plane60, 410, 412. For example, a large increase in the gain would result ina relatively large movement of the object plane 60 in the downward(i.e., negative Z) direction, while a small decrease in the gain wouldresult in a relatively small movement of the object plane 60 in theupward (i.e., positive Z) direction. In this manner, the geometryutilized in the laminographic system of the present invention furtherallows various planes in the object 414 to be imaged upon the detector316 without mechanical movement of any of the system components.

[0075] It will be understood that different configurations of the targetanode 324 may be used in accordance with the present invention. Forexample, FIG. 8 illustrates an embodiment of a target anode that may beused in accordance with the present invention. FIG. 8 shows across-sectional view of this embodiment of the target. In the embodimentshown in FIG. 8, a target 550 comprises multiple concentric rings whichare formed so that X-rays 560 are produced when the electron beam 330 isincident upon the surface of the target 550. Each of the rings has adifferent radius so that objects in different focal planes along theZ-axis are imaged when the electron beam 330 is deflected to trace apath on selected ones of the rings of the target 550.

[0076] Laminographic and Magnification Geometries

[0077]FIG. 1 shows the parameters referred to in the followingdiscussion and equations regarding the laminographic geometry of thepresent method and device. The radius of the circular path followed bythe rotating X-ray detector 30 is “R₀” and maintains a constant value.Similarly, the Z-axis distance between the rotating X-ray source 20(X-ray tube target) and the rotating X-ray detector 30 is “Z₀” andmaintains a constant value. The radius of the circular path followed bythe rotating source of X-rays 20 is “r” and is a variable in thegeometry used for the present invention. The central X-ray path 50 fromthe X-ray source 20 forms an angle “θ” with the common axis of rotation40. The Z-axis distance between the image plane 60 in object 10 and theX-ray detector 30 is “z”. The distance “z” is determined by theintersection 70 of the central X-ray path 50 with the common axis ofrotation 40. Thus, a change in the radius “r” of the circular pathfollowed by the rotating source of X-rays 20 also results in changes inthe angle “θ”, the Z-axis location of the image plane 60, i.e., thepoint of intersection 70, and the Z-axis distance “z” between the imageplane 60 and the X-ray detector 30. The equations to determine theradius “r” required for a specified distance “z” are straightforward asfollows:

z=R ₀/tan θ=(R ₀ /r)(Z ₀ −z)  (1)

[0078] Solving equation (1) for the radius “r” in terms of the Z-axisdistance “z” results in the following: $\begin{matrix}{r = {{{- \left( {R_{0}/z} \right)}\left( {Z_{0} - z} \right)} = {R_{0}\frac{\left( {Z_{0} - 1} \right)}{Z}}}} & (2)\end{matrix}$

[0079] In one configuration of the present invention, the radius “r” ofthe circular path followed by the rotating source of X-rays 20 isadjusted in accordance with equation (2) to electronically change theposition of the Z-axis location “z” of the image plane 60 with respectto the X-ray detector 30. This results in a laminography system whichdoes not require a mechanical system to change the Z-axis location “z”of the image plane 60 with respect to the X-ray detector 30. Forexample, in the laminographic inspection of a circuit board,cross-sectional images of different Z-axis positions of the circuitboard (including both top and bottom surfaces and any other slices), maybe brought into the image plane 60, by electronically shifting theradius of rotation “r” of the rotating source of X-rays 20 as opposed tomechanically moving the circuit board in the Z-axis direction.

[0080] Changing the Z-axis position “z” of the circuit board image plane60 by changing the radius of rotation “r” of the rotating source ofX-rays 20, also results in changing the Field of View (FOV) of the imageformed on detector for different values of “r” and “z”. As previouslydiscussed, the phrase “Field of View” or “FOV” as used herein refers tothe size of a particular region or area of a circuit board which isincluded in a laminographic image of that particular region or area ofthe circuit board. Thus, the size of the image changes with respect tothe region or area of the circuit board being inspected, i.e., themagnification of the image changes when “r” and “z” change. This changein FOV and hence magnification factor must be accurately and efficientlyaccounted for in a circuit board inspection laminography system.

[0081] There are several ways that these changes in FOV withmagnification can be accounted for and corrected in analyzing theimages. In circuit board inspection systems, CAD data which describesthe circuit board being inspected is utilized during the acquisition andanalysis of the images of the circuit board. Thus, a first technique forcompensating for variable image magnification factors and FOV's may beaccomplished by magnifying or shrinking the acquired images to a“nominal” size (“nominal” being defined by a base FOV). Numerousalgorithms for doing this are well documented in the technicalliterature. However, these techniques tend to be CPU intensive and mayaffect throughput of the system. A second and preferred technique forcompensating for variable image magnification factors and FOV's may beaccomplished more efficiently by using on-the-fly CAD data manipulationand on-the-fly FOV adjustments during the analysis of the images.

[0082]FIG. 9 illustrates how the magnification of an image is related tothe distance “z” between the image plane 60 and the X-ray detector 30.Image magnification is defined as the ratio of the size of the image tothe size of the object which forms the image. For example, FIG. 9 showsan object being imaged in the form of an arrow having a linear dimension“2a” in the image plane 60. The image of the arrow is shown in the planeof the X-ray detector 30 and has a linear dimension “2A”. Thus, themagnification is given by “A/a”. The arrow object is positioned in theXZ plane such that the common axis of rotation 40 bisects the arrow inthe X-axis direction. Thus, half the length of the arrow, “a”, lies on afirst side of the axis 40 and the other half lies on a second side ofthe axis 40. The following equations show how the magnification of theimage is related to geometric parameters of the inspection system. Aspreviously shown in FIG. 1, the radius of the path followed by the X-raysource 20 about the common axis of rotation 40 is “r”, the angle formedbetween a central ray 50 from X-ray source 20 and the common axis ofrotation is “θ”, the Z-axis distance between the X-ray source 20 and theplane of the X-ray detector 30 is “Z₀”, the Z-axis distance between theimage plane 60 and the plane of the X-ray detector 30 is “z”, and theZ-axis distance between the X-ray source 20 and the image plane 60 is“(Z₀ z)”. A reference angle “φ” with respect to the vertical axis ofrotation 40 is formed between an X-ray projection 52 from the X-raysource 20 to a first end of the arrow object. The derivation of themagnification of the image, “A/a”, in terms of “A”, “a”, “Z₀”, and “z”is as follows: $\begin{matrix}{{\tan \quad \varphi} = {\frac{a + r}{Z_{0} - z} = \frac{A + R_{0} + r}{Z_{0}}}} & (3)\end{matrix}$

 (a+r)Z ₀=(A+R ₀ +r)(Z ₀ −Z)  (4)

a Z ₀ +r Z ₀ =A(Z ₀ −z)+R ₀(Z ₀ −z)+r(Z ₀ −z)  (5)

aZ ₀ =A(Z ₀ −z)+R ₀(Z ₀ −z)−rz  (6)

[0083] Using equation (2) for “r” in terms of “z”, equation (6) becomes:

a Z ₀ =A(Z ₀ −z)+R ₀(Z ₀ −z)−R ₀(Z ₀ /z−1)  (7)

[0084] Which results in a magnification factor A/a of: $\begin{matrix}{\frac{A}{a} \cdot \frac{Z_{0}}{Z_{0} - z}} & (8)\end{matrix}$

[0085] It is to be understood that the above discussion, althoughpresented in terms of one dimension for purposes of illustration,applies equally to the second dimension of the image plane 60 and theplane 64 of the X-ray detector 30. Since square or rectangularelectronic detectors are more readily available than circular detectors,corresponding square or rectangular images are selected for analysis.Additionally, square or rectangular patterns are more readily adapted tocomputer analysis than are other shapes, e.g., circular patterns.However, the present invention is also applicable to systems which usedetectors and images which are not square or rectangular, includingcircular detectors and images.

[0086] The following specific example of a system having two pre-definedmagnifications further illustrates the geometry discussed above. In thisexample, a square portion of the image formed on the X-ray detector 30is selected. The selected square image has a length and a width of “2A”,which in this example is selected to be approximately equal to 3.8inches. The FOV corresponding to the 2A by 2A (3.8 inches×3.8 inches)image, i.e., the particular region in the image plane 60 of the objectbeing inspected, e.g., a circuit board, has a length and width of “2a”,the size of which varies with the size of the radius “r”. Other fixeddimensions for this example include the radius of the circular pathfollowed by the rotating X-ray detector 30, “R₀”, which is selected tobe approximately equal to 5.8 inches, and the Z-axis distance betweenthe rotating X-ray source 20 (X-ray tube target) and the rotating X-raydetector 30, “Z₀”, which is selected to be approximately equal to 12.5inches. A first magnification factor, MAG 1, of approximately 19× isachieved at a radius “r” of approximately 0.32 inches. The firstmagnification factor, MAG 1, has an FOV of approximately of 0.2inches×0.2 inches in image plane 60 and a Z-axis distance between theimage plane 60 and the plane 64 of the X-ray detector 30 “z” ofapproximately 11.84 inches. Similarly, a second magnification factor,MAG 2, of approximately 4.75× is achieved at a radius “r” ofapproximately 1.55 inches. The second magnification factor, MAG 2, hasan FOV of approximately of 0.8 inches×0.8 inches in image plane 60 and aZ-axis distance between the image plane 60 and the plane 64 of the X-raydetector 30, “z”, of approximately 8.87 inches. The MAG 1 and MAG 2configurations are summarized in Table 1 below. TABLE 1 MAG 1 MAG 2Field of View - FOV (2a × 2a) 0.2″ × 0.2″ 0.8″ × 0.8″ Z-axis Distancebetween Image 0.32″ 1.55″ plane 60 X-ray Detector Plane 30 (z)Magnification Factor (A/a) 19× 4.75× Detected Image Size (2A × . 2A)3.8″ × . 3.8″ 3.8″ × 3.8″ Rotating X-ray Detector Radius (R₀₎ 5.8″ 5.8″Distance between X-ray Source and 12.5″ 12.5″ X-ray Detector (Z₀)

[0087] Circuit Board Inspection Using Electronic Z-axis LaminographySystems

[0088] There are two primary options for using the above describedelectronic Z-axis laminography system for inspection of electricalconnections (e.g., solder joints) on circuit boards. The first optionsupports the circuit board at a single fixed Z-axis position in thesystem and varies the radius of the X-ray source to obtain laminographicimages at all other Z-axis locations of interest. The second optionprovides mechanical supports for the circuit board at multiple fixedZ-axis positions in the system and varies the radius of the X-ray sourceto obtain laminographic images at Z-axis locations intermediate thefixed locations.

[0089] The first option includes a laminography system having amechanical support for the circuit board which is located at a single,i.e., fixed, Z-axis position in the system. While the circuit boardsupport does not allow for movement of the circuit board along theZ-axis of the system, it does provide for precise positioning of thecircuit board along the X and Y axes of the system, wherein the XY planeis substantially parallel to the plane of the circuit board. In thissystem, one base or “central” FOV in a single fixed image plane 60corresponding to the fixed Z-axis location is selected. Laminographicimages of portions of the electrical connections on the circuit boardwhich are either above or below the fixed Z-axis image plane areacquired by changing the radius of the rotating X-ray source asdescribed above. Thus, perturbations to the fixed base or central FOV atthe fixed Z-axis location allows for acquisition of laminographiccross-sectional images of regions above and below the fixed Z-axisposition.

[0090] An example of the first option is illustrated in FIG. 10. Thecircuit board is positioned in the laminography system such that anapproximate midpoint thickness level of the region of the circuit board(or electrical connection on the circuit board) being inspectedapproximately coincides with the fixed image plane referred to as Level1 in FIG. 10. The Z-axis distance between the rotating X-ray sourcepositions 20.1, 20.2, 20.3 and the plane 64 of the X-ray detector 30 is“Z₀”, the Z-axis distance between the image plane 60 and the plane 64 ofthe X-ray detector 30 is “z”, and the Z-axis distance between the X-raysource positions 20.1, 20.2, 20.3 and the image plane 60 is “(Z₀-z)”.The base or “central” FOV in fixed image plane 60, i.e., Level 1, islocated at a distance z₁, from the plane 64 of the X-ray detector 30 andis characterized by: a first magnification factor; a first radius“r_(l),” of the rotating X-ray source 20.1; a first FOV havingdimensions of 2a₁,×2a₁, in image plane 60; and a Z-axis distance betweenthe image plane 60 and the plane of the rotating X-ray source 20.1 of“(Z₀₋z₁,)”. A second FOV, i.e., Level 2, is located above Level 1 at adistance Z₂ from the plane 64 of the X-ray detector 30 and ischaracterized by: a second magnification factor; a second radius “r₂” ofthe rotating X-ray source 20.2; a second FOV having dimensions of2a₂×2a₂ in image plane 60; and a Z-axis distance between the image plane60 and the plane of the rotating X-ray source 20.2 of “(Z₀-z₂)”. A thirdFOV, i.e., Level 3, is located below Level 1 at a distance Z₃ from theplane 64 of the X-ray detector and is characterized by: a thirdmagnification factor; a third radius “r₃” of the rotating X-ray source20.3; a third FOV having dimensions of 2a₃×2a₃ in image plane 60; and aZ-axis distance between the image plane 60 and the plane of the rotatingX-ray source 20.3 of “(Z₀-Z₃)”. Similarly, laminographic images atlevels intermediate Levels 1 and 2 and Levels 1 and 3 may be acquired byselecting the appropriate radius of the rotating X-ray source 20 inaccordance with equation (2).

[0091] Applying the above example to a typical circuit board furtherillustrates this system. In this specific example, the base or “central”FOV, is selected to correspond to the specific system configurationreferred to as MAG 2 as summarized in Table 1. The Z-axis levels ofinterest for a typical electrical connection inspection are generallylocated within a range of approximately ±60 mils (0.060 inch) centeredabout a central Z-axis level. Equation (2) is used to derive the valueof the X-ray source radius for each specific Z-axis level. Equation (8)is used to derive the value of the Magnification factor for eachspecific Z-axis level. Examples of specific parameters for Levels 1, 2and 3 of this configuration are presented in Table 2. Laminographicimages at levels intermediate Levels 1 and 2 and Levels 1 and 3 may beacquired by selecting the appropriate radius of the rotating X-raysource 20 between 1.50 inches and 1.59 inches in accordance withequation (2). Similarly, laminographic images at levels above Level 2and/or below Level 3 may be acquired by selecting the appropriate radiusof the rotating X-ray source 20 in accordance with equation (2). TABLE 2Level 2 Level 1 Level 3 FOV (2a × 2a) 0.78″ × 0.78″ 0.80″ × 0.80″ 0.82″× 0.82″ X-ray Radius (4) 1.50″ 1.55″ 1.59″ Image Plane-to- 9.93″ 9.87″9.81″ Detector (z) Magnification Factor 4.86× 4.75× 4.65× (A/a) ImageSize (2A × 2A) 3.8″ × . 3.8″ 3.8″ × . 3.8″ 3.8″ × . 3.8″ X-ray DetectorRadius 5.8″ 5.8″ 5.8″ (R₀₎ X-ray Source-to- 12.5″ 12.5″ 12.5″ Detector(Z₀₎

[0092] The second option includes a laminography system which providesmechanical supports for the circuit board at multiple fixed Z-axispositions in the system and varies the radius of the X-ray source toobtain laminographic images at Z-axis locations intermediate the fixedlocations. An example of the second option involves a simplified Z-axisthat allows 2 or more discreet “stops” so that multiple FOVs can be usedfor magnification purposes. For example, fine pitch devices oftenrequire higher magnification than larger discreet components likepassive devices (e.g., chip capacitors and resistors). Using thespecific example illustrated in Table 1, this type of system could havea first fixed position which magnifies the image by a factor of 4.75 forinspections of the large components on the circuit board and a secondfixed position which magnifies the image by a factor of 19 forinspections of the smaller fine pitch devices on the circuit board.Thus, this simplified dual-position design still provides forinspections at any value of Z within the designed inspection range ofthe system, but no longer requires on-the-fly accurate high-speed Zpositioning at any continuous value of Z within the designed inspectionrange of the system.

[0093] On-the-fly CAD Data Manipulation

[0094] Circuit board inspection systems typically include data filesthat describe the positions, size, pin locations, and other importantdesign data for all inspected solder joints and other features in allboard views. As previously stated, the phrase “board view” refers to thelaminographic image of a particular region or area of the circuit boardidentified by a specific X, Y coordinate of the circuit board. Acomplete inspection of a circuit board typically includes multiple boardviews. Additionally, some board views include multiple slices, i.e.,cross-sectional images acquired at different Z height locations orlayers of the circuit board.

[0095] In prior art inspection systems, the image plane of theinspection system may include several fixed Z-axis locations, one foreach calibrated FOV, and the multiple image slices of the circuit boardwithin one of the FOV's at different Z-levels with respect to thecircuit board are acquired by mechanically moving the circuit boardalong the inspection system Z-axis such that the desired Z-level sliceof the circuit board coincides with the fixed inspection system Z-axislocation of the image plane for that FOV. Since the inspection systemZ-axis location of the image plane is fixed, and the multiple imageslices of the circuit board at different Z-levels with respect to thecircuit board are positioned at this fixed inspection system Z-axislocation, all of the images acquired at this fixed inspection systemZ-axis location have the same FOV and magnification. However, aspreviously described, the present invention replaces the mechanicalmovement of the circuit board along the Z-axis of the inspection systemwith electrically controlled relocation of the image plane along theZ-axis of the inspection system. As previously described, this resultsin image planes positioned at different Z-axis locations of theinspection system having different FOV's and magnifications. The presentinvention compensates for these changes in FOV and magnification bymodifying the CAD design data on-the-fly to match the magnification ofthe current image.

[0096] In the present context, “on-the-fly” data analysis refers to realtime or near real time modification of the CAD data on as-needed basisfor analysis of the current image as opposed to modifying the CAD datain advance and storing it for later recall and utilization. Analysis ofthe image is then carried out in the normal manner, i.e., comparing theacquired image data with the CAD design data, using the modified CADdata. For example, one situation where on-the-fly CAD data modificationis advantageous is where surface mapping of the boards before inspectionreveals that each new circuit board is likely to have different Zheights with respect to the inspection system for the same specific Zlevel within a specific solder joint. These variations in Z heights withrespect to the inspection system from board to board are most commonlydue to variations in board warpage measured by laser range finderreadings from board to board. In other words, board warpage causes a Zlevel referenced to the circuit board surface to be located at differentZ-axis levels of the laminographic inspection system. Thus, electronicrelocation of the image plane with respect to the inspection systemcoupled with on-the-fly CAD data modification provides a processor timeand data storage resource efficient means for acquiring images at thedesired Z levels of the circuit board and analyzing these images byrecalling, modifying and applying the CAD data for use on an as neededbasis.

[0097] On-the-fly CAD data manipulation requires that various CAD datafields be modified. The CAD data fields requiring modification dependson the current FOV. Examples of specific CAD files which often requiremodification are discussed below. The CAD data files discussed below areincluded to illustrate the procedure and are not to be considered aslimiting which files may be subject to modification when practicing thepresent invention. The present invention is applicable to virtually anytype of CAD data which may be required for laminographic electricalconnection or solder joint inspection.

[0098] Conversions: Pixels to Mils and Mils to Pixels

[0099] Conversion of image units, e.g., pixels, to physicaldimensionals, e.g., mils, is accomplished by dividing the physical sizeof the current FOV by the number of pixels included in an image framebuffer width as follows:

PixelsToMils=Current FOV in mils/Frame Buffer Width in pixels  (9)

[0100] This is illustrated by a specific example where the number ofpixels in an image is 2048×2048 and the image corresponds to a region ofan object having physical dimensions of 800 mils×800 mils (0.8inches×0.8 inches). The PixelsToMils conversion factor for this exampleis determined from equation (9) by dividing 800 mils by 2048 pixels,yielding a PixelsToMils conversion factor which is approximately equalto 0.391 mils. Thus, the width of each pixel in the image corresponds toa width of approximately 0.391 mils on the object shown in the image.

[0101] Similarly, the inverse conversion from physical dimensionals,e.g., mils, to image units, e.g., pixels, is accomplished by dividingthe number of pixels included in an image frame buffer width by thephysical size of the current FOV as follows:

MilsTopixels=Frame Buffer Width in pixels/Current FOV in mils  (10)

[0102] Using the previous specific example where the number of pixels inthe image is 2048×2048 and the image corresponds to a region of anobject having physical dimensions of 800 mils×800 mils, the MilsToPixelsconversion factor is determined from equation (10) by dividing 2048pixels by 800 mils, yielding a MilsToPixels conversion factor which isapproximately equal to 2.56 pixels. Thus, a width of approximately 1 milon the object corresponds to approximately 2.56 pixels in the image ofthe object.

[0103] Conversion of CAD Coordinates to Pixels

[0104] It is generally advantageous to perform on-the-fly calculationsfor coordinating and comparing CAD data with image data in pixelcoordinate format. However, the primary format for CAD data received bythe inspection facility is generally in physical dimensions format(e.g., mils). Thus, the physical dimensions format CAD data is convertedto pixel format using equation for the conversion of Mils-To-Pixels.Since coordination and comparison of the CAD data with the image data isgenerally performed in pixel coordinate format, the following discussionis presented in terms of pixel coordinate format. However, if physicaldimensions format is preferred for a particular application, the presentinvention may also be practiced in physical dimension format.

[0105] FOV Correction Factor

[0106] A “nominal FOV” or reference FOV refers to a field of view whichis used as a reference for calibration of other FOV's of thelaminography inspection system. For example, a laminography inspectionsystem may be configured such that the nominal/reference FOV correspondsto an image having a specific magnification factor of an area in theimage plane having a specific size. In certain situations, it may beadvantageous for a laminography inspection system to have multipleconfigurations and corresponding multiple nominal FOVs or referenceFOVs. For example, referring to the example of a specific laminographysystem summarized in Table 1, this system has a first nominal/referenceFOV (MAG 1) which creates a 3.8 inch×3.8 inch image on 5 a detectorwhich corresponds to an area in the image plane which is 0.2 inch×0.2inch in size. Thus, this first nominal/reference FOV has a magnificationfactor of 19. Similarly, this system also has a second nominal/referenceFOV (MAG 2) which creates a 3.8 inch×3.8 inch image on a detector whichcorresponds to an area in the image plane which is 0.8 inch×0.8 inch insize. Thus, this second nominal/reference FOV has a magnification factorof 4.75.

[0107] The fields of view and magnification factors for Z-axis locationswhich are different from the Z-axis location for a nominal/reference FOVare referred to in this discussion as a “current FOV”. Thus, if an imageis acquired at a “current FOV” which does not coincide with a “nominalFOV”, the CAD data must be adjusted to reflect the differences (e.g.,magnification, etc.) between the nominal FOV and the current FOV beforethe image data in the current FOV can be compared to the CAD designdata. This FOV conversion of the CAD data from a nominal FOV to acurrent FOV is done on-the-fly and uses a conversion factor referred toas “FOV Correction” which is calculated as follows:

FOVCorrection=NominalFOV/CurrentFOV  (11)

[0108] A specific example, shown in FIGS. 11, 12, 13 and 14, is used inthe following discussion to illustrate the application of on-the-fly CADdata modification for analysis of laminographic circuit board imagesusing the FOVCorrection conversion factor, location conversion factors,and length conversion factors. FIGS. 11 and 12 show a perspective viewand a cross sectional view, respectively, of the test object 10 (seeFIG. 2a) mounted on a circuit board 620. Three corners of the testobject 10 are located at circuit board coordinates (x₁,y₁,), (x₂,y₁,)and (x₁,y₂). The center of the test object 10 is located at circuitboard coordinates (x_(Pc),y_(Pc)). Referring to FIGS. 10 and 12, thecircle image plane 60 b in the test object 10 is located at the distancez, from the plane 64 of the X-ray detector 30 (Level 1); the arrow imageplane 60 a in the test object 10 is located at the distance Z₂ from theplane 64 of the X-ray, detector 30 (Level 2); and the cross image plane60 c in the test object 10 is located at the distance Z₃ from the plane64 of the X-ray detector 30 (Level 3). As shown in FIG. 11, coordinates(x_(BV),y_(BV)) identify a first board view location on circuit board620.

[0109] For purposes of this example, the test object 10 has beenselected to have the following physical characteristics: a length ofapproximately 413 mils; a width of approximately 213 mils; a height ofapproximately 240 mils; the circle image plane 60 b (Level 1) positionedin the middle of the height dimension; the arrow image plane 60a (Level2) positioned 60 mils above Level 1; and the cross image plane 60 c(Level 3) positioned 60 mils below Level 1. Additionally, thelaminography system selected for the inspection of the test object 10having these physical characteristics is the specific example systemsummarized in Table 2.

[0110] As previously stated, it is often more efficient to store thedesign CAD data for a particular circuit board in pixel format. In oneimplementation of the present invention, it has been found thaton-the-fly calculation times can be minimized if the CAD data is storedin the analysis system in a pixel format which corresponds to a specificnominal/reference FOV. Additionally, inspection procedures may bedefined for each particular circuit board. These inspection proceduresinclude defining specific board views and objects or features (e.g.,solder joints) to inspect in each board view.

[0111] For example, one inspection procedure designed to check theposition and dimensions of the test object 10 on circuit board 620 andthe features of the circle 82, arrow 81 and cross 83 in the test object10 at Levels 1, 2 and 3, respectively, includes the following steps.First, determine a first board view location at board coordinates(x_(Bv),y_(Bv)) such that Level 1, 2 and 3 board views at this locationinclude the test object 10. Second, define a first board view centeredat board coordinates (x_(Bv),y_(Bv)) at the first Z-axis level z₁,(Level 1) and select the FOV of the first board view as the nominal FOV.Third, define a second board view centered at board coordinates(x_(BV),y_(BV)) at the second Z-axis level Z2 (Level 2) and select theFOV of the second board view as the first current FOV. Fourth, define athird board view centered at board coordinates (x_(BV),y_(Bv)) at thethird Z-axis level Z₃ (Level 3) and select the FOV of the third boardview as the second current FOV.

[0112] Implementation of this procedure to check the position anddimensions of the test object 10 on circuit board 620 and the featuresof the circle 82, arrow 81 and cross 83 in the test object 10 at Levels1, 2 and 3, respectively, includes creating a pixel format CAD databasewhich describes the features of the test object 10 from the physicaldimensions CAD database for the test object 10. An example of a pixelformat CAD database 720 cd corresponding to the first board viewcentered at board coordinates (x_(BV),y_(BV)) at the first Z-axis levelz₁, (Level 1) and the nominal FOV is shown in FIG. 13A. The CAD data 720cd for Level 1 shows the center of the first board view, correspondingto the first board view dimensional coordinates (x_(BV),y_(BV)), locatedat pixel coordinates 0 024,1024). Additionally, the CAD data 720 cd forLevel 1 shows:

[0113] a) the three corners of the test object 10 corresponding to thedimensional coordinates (x₁y₂), (x₁,y₁), and (x₂,y₁), located at pixelcoordinates (195,920), 0 253,920) and (1253,375), respectively; and b)the center of the test object corresponding to the dimensionalcoordinates (x_(PC),y_(PC)), located at pixel coordinates (724,648).

[0114] A first laminographic image or board view 720 id corresponding tothe first board view location (x_(BV),y_(BV)) at the first Z-axis levelz, (Level 1) of test object 10 is shown in FIG. 14A. The image data 720id for Level 1 shows the center of the first board view, correspondingto the first board view dimensional coordinates (x_(BV),y_(BV)), locatedat image pixel coordinates (1024,1024). The field of view (FOV) of thefirst board view image data 720 id at Level 1, i.e., the portion of thecircuit board 620 at the first Z-axis level z₁, centered at the firstboard view location (x_(BV),y_(BV)) which is included in the firstlaminographic board view image 720 id, is represented by the dashed lineperimeter 640 in FIG. 11.

[0115] In this example, the FOV 640 at the dashed line position isselected as the “nominal FOV and has a size of approximately 800mils×800 mils (see Table 10).

[0116] In a typical circuit board inspection system according to thepresent invention, the CAD data which describes features in the firstfield of view 640 (e.g., the location and dimensions of the test object10, the circles within the plane 60 b of the test object 10, etc.) areavailable to the analysis portion of the inspection system in pixelformat. As shown in the first board view image data 720 id in FIG. 14A:a) the test object 10 forms an image having corners at pixel locations(205,920), (1263,920) and (1263,375) corresponding to the corners(x₁,y₂), (x₁,y₁) and x₂,y₁), respectively, of the test object 10; and b)the center of the test object 10 forms an image at pixel locations(734,648) corresponding to the center (x_(PC),y_(PC)) of the test object10. Since the FOV 640 was selected as the “nominal FOV”, the first boardview image data at level z₁, 720 id (FIG. 14A) may be compared directlyto the pixel format test object 10 CAD data at level z₁, (FIG. 13A)corresponding to the first board view centered at board coordinates(x_(Bv),y_(Bv)) at the first Z-axis level z₁. This comparison of theimage data 720 id (FIG. 14A) with the corresponding CAD data 720 cd(FIG. 13A) reveals that the test object 10 is shifted in the positive xdirection by 10 pixels and is positioned correctly in the y direction.

[0117] In accordance with the present invention, inspection of the arrowimage plane 60 a (Level 2 at Z₂) positioned 60 mils above Level 1; andthe cross image plane 60 c (Level 3 at Z₃) positioned 60 mils belowLevel 1, is accomplished by changing the radius of the X-ray source assummarized in Table 2. Thus, to change from Level 1 to Level 2, theradius of the X-ray source is changed from approximately 1.55 inches toapproximately 1.50 inches which also changes the FOV from approximately800 mils×800 mils to approximately 780 mils×780 mils. A secondlaminographic image or board view 750 id corresponding to the firstboard view location (x_(Bv),y_(Bv)) at the second Z-axis level Z₂ (Level2) of test object 10 is shown in FIG. 14B. The image data 750 id forLevel 2 shows the center of the first board view, corresponding to thefirst board view dimensional coordinates (x_(BV),y_(BV)), located atimage pixel coordinates (1024,1024). The field of view (FOV) of thefirst board view image data 750 id at Level 2, i.e., the portion of thecircuit board 620 at the second Z-axis level Z₂ centered at the firstboard view location (x_(BV),y_(BV)) which is included in the secondlaminographic board view image 750 id , is selected as the first“current FOV” and has a size of approximately 780 mils×780 mils (seeTable 2). Similarly, to change from Level 1 to Level 3, the radius ofthe X-ray source is changed from approximately 1.55 inches toapproximately 1.59 inches which also changes the FOV from approximately800 mils×800 mils to approximately 820 mils×820 mils. A thirdlaminographic image or board view 780 id corresponding to the firstboard view location (x_(BV),y_(BV)) at the third Z-axis level Z₃ (Level3) of test object 10 is shown in FIG. 14C. The image data 780 id forLevel 3 shows the center of the first board view, corresponding to thefirst board view dimensional coordinates (x_(BV),y_(BV)), located atimage pixel coordinates (1024,1024). The field of view (FOV) of thefirst board view image data 780 id at Level 3, i.e., the portion of thecircuit board 620 at the third Z-axis level Z3 centered at the firstboard view location (x_(BV),y_(BV)) which is included in the thirdlaminographic board view image 780 id, is selected as the second“current FOV” and has a size of approximately 820 mils×820 mils (seeTable 2).

[0118] The FOV Correction factor previously defined in equation 11, isused to perform the on-the-fly conversion of specific X, Y coordinatesin the CAD data from a NominaIFOV to a CurrentFOV as follows:

CurrentX=(NominalX−ImageCenterX)*FOVCorrection+ImageCenterX

CurrentY=(NominalY−ImageCenterY)*FOVCorrection+ImageCenterY  (12)

[0119] The FOVCorrection factor is also used to perform on-the-flyconversion of specific dimensions in the X-axis and Y-axis directions,AX and DY, in the CAD data from a NominaIFOV to a CurrentFOV as follows:

CurrentΔX=NominalΔX*FOVCorrection

CurrentΔY=NominaIΔY*FOVCorrection  (13)

[0120] Examples of on-the-fly conversion of the nominal FOV CAD database720 cd for test object 10 at Level 1 shown in FIG. 13A to the firstcurrent view at Level 2 and the second current view at Level 3 are shownin FIGS. 13B and 13C, respectively. For example, equations 12 convertthe CAD data of Level 2 at the nominal FOV of Level 1 to the firstcurrent FOV of Level 2 (FIG. 13B) as follows: a) the three corners ofthe test object 10 corresponding to the dimensional coordinates (x₁,y₂),(x₁,y₁) and (x₂,y₁), are located at pixel coordinates (176,918),(1258,918) and (1258,360), respectively, in the first current FOV; andb) the center of the test object 10 corresponding to the dimensionalcoordinates (x_(PC),y_(PC)), is located at pixel coordinates (717,639)in the first current FOV. Similarly, equations 12 convert the CAD dataof Level 3 at the nominal FOV of level 1 to the second current FOV ofLevel 3 (FIG. 13C) as follows: a) the three corners of the test object10 corresponding to the dimensional coordinates (x₁,y₂), (x₁,y₁) and(x₂,y₁), are located at pixel coordinates (212,922), (1248,922) and(1248,389), respectively, in the second current FOV; and b) the centerof the test object 10 corresponding to the dimensional coordinates(x_(PC),y_(PC)), is located at pixel coordinates (730,656) in the secondcurrent FOV. Thus, analysis of the laminographic image for Level 2 (FIG.14B) is accomplished using the first current FOV on-the-fly convertedCAD data for Level 2 (FIG. 13B) and analysis of the laminographic imagefor Level 3 (FIG. 14C) is accomplished using the second current FOVon-the-fly converted CAD data for Level 3 (FIG. 13C). Equations 13 areemployed in a similar manner to convert lengths in the CAD data of Level1 (FIG. 13A) at the nominal FOV to the first current FOV of Level 2(FIG. 13B) and the second current FOV of Level 3 (FIG. 13C) forcomparison to the corresponding laminographic images at Levels 1, 2 and3.

[0121] Examples of specific parameters which are often used forinspection of solder joints/electrical connections include padlocations, pin locations, and pad dimensions. On-the-fly conversion ofthe CAD fields/data for these parameters may be accomplished as follows:

PadXLocation=(NominaIPadX−XlmageCenter)*FOVCorrection+XlmageCenter

PadYLocation=(NominaIPadY−YlmageCenter)*FOVCorrection+YlmageCenter  (14)

PinXLocation=(NominaIPinX−XlmageCenterl*FOVCorrection+XlmageCenter

PinYLocation=(NominaIPinY−YlmageCenter)*FOVCorrection+YlmageCenter  (15)

PadDx=NominaIPadDx*FOVCorrection

PadDy=NominaIPadDy*FOVCorrection

PinDx=NominaIPinDx*FOVCorrection

PinDy=NominaIPinDy*FOVCorrection  (16)

[0122] These coordinate translations and scaling are performed for eachslice of each board view for each board inspected, based on the currentZ height which determines the current FOV and magnification.

[0123] As presented in the above discussion and specific examples, thecoordinates of features on the circuit board are referenced to theselected board view. Thus, the same features, referenced to a differentboard view would have different coordinates. Many such issues arise inthe specific application of the present invention and do not limit thescope of the present invention, as its teachings can be readily adaptedto numerous analysis conventions.

[0124] The process for performing on-the-fly CAD data manipulations forthe analysis of laminographic images according to the present inventionis summarized in the flow chart shown in FIG. 15. In block 810, a“Nominal or Reference Image Plane” at a Z-axis location “z₁,” in theobject being inspected is determined. In block 820, the size of the“Nominal FOV” corresponding to the Reference/Nominal Image Plane atZ-axis location “z₁” is determined. In block 830, a “First Current ImagePlane” at a Z-axis location “z_(s)” in the object being inspected isdetermined. In block 840, the size of the First “Current FOV”corresponding to the First Current Image Plane at Z-axis location “Z₂”is determined. In block 850, an FOVCorrection Factor for the FirstCurrent Image Plane is determined by using the sizes of the “NominalFOV”; the First “Current FOV” and Equation (11). In block 860, theFOVCorrection Factor is used to adjust the CAD data on-the-fly as neededfor the analysis of the cross-sectional laminographic image at Z-axislocation “Z₂”. While the above discussion has been in terms of anFOVCorrection factor based on relative sizes of the fields of view atdifferent Z-axis locations, a similar and equivalent procedure couldalso be performed using other parameters, for example, the magnificationfactors for the different Z-axis locations. These and other suchmodifications are considered to be included within the scope of thepresent invention.

[0125] Other Issues and Considerations

[0126] Since the present system supplies cross-sectional images atvarious Z heights, a common use is to take multiple slices of a solderjoint and correlate data between slices. Although the distances betweenslices are generally small, a few mils, for most surface mount devices,more substantial distances can be encountered for certain specificdevice types. For example, BGA devices are sometimes imaged at both thetop and bottom of the ball, which is a distance of approximately 25mils. Plated Through Hole (PTH) devices are imaged on both the top andbottom pads, which will be the entire board thickness, which is adistance of approximately 70 mils. For these larger distances, somemeasurements must be corrected.

[0127] For example, a locator algorithm is usually run at the center ofthe ball for BGA devices. The x and y coordinates it finds willcorrespond to different x and y locations on different slices, such asthe pad slice and the top package slice.

[0128] Similarly a PTH location found in the barrel may need to beadjusted on the top and bottom pad. These can be cleverly handled bycorrecting these locations in a software module, which maintains anddistributes these located positions to the algorithms. Similarmagnification corrections to those discussed above to correct CADlocations can be applied to the locator positions at runtime as well.

[0129] Similarly, size measurement may need normalization depending onthe Z-height of the slice used to gather the measurement. For example,any measurements in units of pixel distances, if they exist, must beconverted to mils using the current perturbed FOV before any comparisonor use on different slices.

Summary, Ramifications and Scope

[0130] Accordingly, the reader will see that the present inventionsolves many of the specific problems encountered when inspecting solderconnections on circuit boards. Particularly important is that it removesthe need for a mechanical means to move the circuit board along theZ-axis without impeding the analysis of the laminographic images atvarious Z-axis levels in the circuit board. Furthermore, the electronicrelocation of the image plane with respect to the inspection systemcoupled with on-the-fly CAD data modification provides a processor timeand data storage resource efficient means for acquiring images at thedesired Z levels of the circuit board and analyzing these images byrecalling, modifying and applying the CAD data for use on an as neededbasis.

[0131] Although the description above contains many specificities, theseshould not be construed as limiting the scope of the invention but asmerely providing illustrations of some of the presently preferredembodiments of this invention. For example, alternative techniques andimage parameters may be used to determine how to convert the CAD data atthe nominal FOV for use at a current FOV. Additionally, alternative CADdata parameters may be used for image analysis; alternative techniquesmay be used to acquire the cross sectional images; alternative methodsmay be used for changing the Z-axis level at which images are acquired;etc.

[0132] The invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims and their legal equivalents, rather than by theforegoing description and specific examples given. All changes whichcome within the meaning and range of equivalency of the claims are to beembraced within their scope.

Therefore, having thus described the invention, at least the followingis claimed:
 1. A processor configured to control the acquisition andanalysis of cross-sectional images of an object in an x-ray inspectionsystem, each cross-sectional image corresponding to a Z-axis position,the processor programmed to perform the steps of: receiving a currentZ-axis position of a current cross-section of an object to be inspected,the current Z-axis position defining a Z-axis variance relative to areference Z-axis position of a reference cross-section of the object;based on the Z-axis variance, generating a current cross-sectional imageof the object at the current Z-axis position; and based on the Z-axisvariance, modifying design data that models the reference cross-sectionof the object.
 2. The processor of claim 1, wherein the processor isfurther programmed to perform the step of comparing the currentcross-sectional image of the object to the modified design data.
 3. Theprocessor of claim 1, wherein the step of modifying the design databased on the Z-axis variance further comprises converting the designdata into a pixel coordinate format.
 4. The processor of claim 1,wherein the object being inspected comprises a printed circuit board. 5.The processor of claim 1, wherein the step of modifying the design databased on the Z-axis variance comprises manipulating the design dataon-the-fly.
 6. The processor of claim 1, wherein the step of generatinga current cross-sectional image of the object further comprisesdetermining a current field of view (FOV) based on the Z-axis variance.7. The processor of claim 1, wherein the step of generating a currentcross-sectional image of the object further comprises determining acurrent magnification factor based on the Z-axis variance.
 8. Theprocessor of claim 6, wherein the step of modifying the design datafurther comprises modifying the design data based on a comparisonbetween the current FOV and a reference FOV associated with thereference Z-axis position.
 9. The processor of claim 7, wherein the stepof modifying the design data further comprises modifying the design databased on a comparison between the current magnification factor and areference magnification factor associated with the reference Z-axisposition.
 10. An x-ray inspection system for acquiring and analyzingcross-sectional images of an object, the x-ray inspection systemcomprising: a source of x-rays configured to emit x-rays through anobject to be inspected; an x-ray detector system configured to receivethe x-rays produced by the source of x-rays and generate across-sectional image of a portion of the object being; logic configuredto receive a current Z-axis position of a current cross-section of anobject to be inspected, the current Z-axis position defining a Z-axisvariance relative to a reference Z-axis position of a referencecross-section of the object; logic configured to generate, based on theZ-axis variance, a current cross-sectional image of the object at thecurrent Z-axis position; and logic configured to modify, based on theZ-axis variance, design data that models the reference cross-section ofthe object.
 11. The x-ray inspection system of claim 10, furthercomprising logic configured to compare the current cross-sectional imageof the object to the modified design data.
 12. The x-ray inspectionsystem of claim 10, wherein the logic configured to modify the designdata based on the Z-axis variance further comprises logic configured toconvert the design data into a pixel coordinate format.
 13. The x-rayinspection system of claim 10, wherein the object being inspectedcomprises a printed circuit board.
 14. The x-ray inspection system ofclaim 10, wherein the logic configured to modify the design data basedon the Z-axis variance further comprises logic configured to manipulatethe design data on-the-fly.
 15. The x-ray inspection system of claim 1,wherein the logic configured to generate a current cross-sectional imageof the object further comprises logic configured to determine a currentfield of view (FOV) based on the Z-axis variance.
 16. The x-rayinspection system of claim 10, wherein the logic configured to generatea current cross-sectional image of the object further comprises logicconfigured to determine a current magnification factor based on theZ-axis variance.
 17. The x-ray inspection system of claim 15, whereinthe logic configured to modify the design data further comprises logicconfigured to modify the design data based on a comparison between thecurrent FOV and a reference FOV associated with the reference Z-axisposition.
 18. The x-ray inspection system of claim 16, wherein the logicconfigured to modify the design data further comprises logic configuredto modify the design data based on a comparison between the currentmagnification factor and a reference magnification factor associatedwith the reference Z-axis position.
 19. The x-ray inspection system ofclaim 10, wherein the logic comprises software residing in memory andfurther comprising a processor configured to implement the logic.
 20. Amethod for acquiring and analyzing cross-sectional images in an x-rayinspection system, the method comprising the steps of: receiving acurrent Z-axis position of a current cross-section of an object to beinspected, the current Z-axis position defining a Z-axis variancerelative to a reference Z-axis position of a reference cross-section ofthe object; based on the Z-axis variance, generating a currentcross-sectional image of the object at the current Z-axis position; andbased on the Z-axis variance, modifying design data that models thereference cross-section of the object.
 21. The method of claim 20,further comprising the step of comparing the current cross-sectionalimage of the object to the modified design data.
 22. The method of claim20, wherein the step of modifying the design data based on the Z-axisvariance further comprises converting the design data into a pixelcoordinate format.
 23. The method of claim 20, wherein the object beinginspected comprises a printed circuit board.
 24. The method of claim 20,wherein the step of modifying the design data based on the Z-axisvariance comprises manipulating the design data on-the-fly.
 25. Themethod of claim 20, wherein the step of generating a currentcross-sectional image of the object further comprises determining acurrent field of view (FOV) based on the Z-axis variance.
 26. The methodof claim 20, wherein the step of generating a current cross-sectionalimage of the object further comprises determining a currentmagnification factor based on the Z-axis variance.
 27. The method ofclaim 25, wherein the step of modifying the design data furthercomprises modifying the design data based on a comparison between thecurrent FOV and a reference FOV associated with the reference Z-axisposition.
 28. The method of claim 26, wherein the step of modifying thedesign data further comprises modifying the design data based on acomparison between the current magnification factor and a referencemagnification factor associated with the reference Z-axis position. 29.A system for acquiring and analyzing cross-sectional images of an objectin an x-ray inspection system, system comprising: a means for receivinga current Z-axis position of a current cross-section of an object to beinspected, the current Z-axis position defining a Z-axis variancerelative to a reference Z-axis position of a reference cross-section ofthe object; a means for generating, based on the Z-axis variance, acurrent cross-sectional image of the object at the current Z-axisposition by emitting x-rays through the current cross-section of theobject and generating a corresponding x-ray image; and a means formodifying, based on the Z-axis variance, design data that models thereference cross-section of the object.
 30. The system of claim 29,further comprising a means for comparing the current cross-sectionalimage of the object to the modified design data.